Heat oven to 190C/fan 170/gas 5. Slice the potatoes and boil for 6-8 mins until just tender, then drain.
Butter or oil one large or two smaller shallow ovenproof dishes that will fit the fish fillets in one layer (a couple of roasting tins would work well). Scatter the potatoes over the base of the dishes, then halve the tomatoes and scatter on top, along with the olives. Set the fish fillets among the other ingredients then pour over the stock and wine, then season to taste.
Cover the dishes with foil and seal the edges. Bake for 15 mins until the fish is tender. Serve each portion on warm plates and scatter with basil.